Network troubleshooting

I have been told that I have a problem on my network where one or
more of the connections (nic cards, switch router,hub, or even
cabling) are causing problems with the integrety of the data
being passed back and forth across our network. Is there a way
to find out what is causing this? ie testing software,or
procedures, or a way to isolate which connection is causing
this. This problem started last week. I have not had this
problem before. If you require more network info plesae let me
know. Thanks in advance for your assistance.
ITnWV

John,
This (network troubleshooting) is an entire discipline. It should be
a common skill set, but sadly not. Still, it's not hard:

1. Start with the symptoms. WHO advanced the theory and why?
2. Now take your system documentation, the traffic map if you will.
Determine if you can any isolations - when and where are the problems
showing up, and not?
3. Then look at instrumentation you may have already - switch/router
statistics, monitoring, etc. Anywhere errors show?
4. Next you WILL need an analyzer. Don't be afraid, these aren't as
geeky as their reputation implies - and it's something all network
professionals should be able to work with, at least at the entry
level. Software analyzers like Ethereal are great, but you won't see
all frame errors with most network adapters; you'll need a hardware
or hardware/software analyzer for that. Insert at suspicious or
interesting locations (at the router, server, or consolidation links
depending on the structure and look for conversation oddities -
retries, timeouts, invalid checksums and frame errors.

This should give you a base idea of what's going on, what's working
and not working well on your network. You may need some assistance in
interpreting the results or it may be so terribly clear that there's
no question about the problem. Now this is the generalized approach;
how well you can apply it depends on your available skill sets, size
and complexity of your network, the degree of the problem and how
much time you have to resolve. You may well gather the initial data
and decide to bring on outside help. You may be able to solve it
yourself, or just need a bit of remote help to sort things out. If
further discussion will help let me know; more information (network
topology, symptoms, etc) will help target advice.